Electron-optical corrector for eliminating third-order aberations

ABSTRACT

The invention relates to an electron-optical corrector for elimanting third-order aberrations, such as spherical aberrations, field curvature and off-axis astigmatism; said corrector being devoid of third-order off-axis coma, third-order distrortion and first-order chromatic aberration of the first degree. The corrector has a construction which is symmetrical about the central plane in the direction of the linear optical axis. A hexapole S 1  of length l 1 is first positioned in the direction of the beam path, followed by a circular lens R l , a hexapole S 2  of length l 2  and subsequently a circular lens R 2  which is followed by a third hexapole S 3  with the same strength with the same strength of the hexapole S l  and double the length of the latter 1 3 =21 3 . The separation of the two circular lenses R 1 , R 2  and the distance from the circular lens to the first hexapole S 1  is chosen in such a way that the internal plane of S 1  comes to rest in the front principal focus of the corcular lens that is positioned downstream and the center of the hexapoles S 2  and S 3  is located on the focal plane. Additional elements of the corrector also follow in sequence, said clements being symmetrical about the cnetral plane Z m  of the hexapole S 3 .

[0001] The invention relates to an electron-optical corrector for eliminating third-order aberrations, such as spherical aberrations, field curvature and off-axis astigmatism; said corrector being devoid of third-order off-axis coma, third-order distortion and first-order chromatic aberrations of the first degree. The corrector has a construction which is symmetrical about the central plane in the direction of the linear optical axis.

[0002] The efficiency of electron-optical systems, which in the sense of this invention are also understood to include those with ion-imaging systems, is limited by their image aberrations, of which, depending on the specific application and the extent of the corrections already made, particular image aberrations are responsible for limiting the performance, the elimination of which represents considerable progress in the improvement of electron-optical systems. It is possible to systematically subdivide and classify the image aberrations into axial image aberrations, which are also determined by the fundamental paths emerging in the two sections of the optical axis in the object plane, off-axis image aberrations, which in turn are dependent on the fundamental paths emerging outside the optical axis in the object plane, and chromatic aberrations, which only occur with different speeds of the imaging electrons. With magnifying electron-optical systems, such as those used in electron microscopy, it is most important to eliminate the axial image aberrations to increase efficiency. With size-reducing electron-optical systems, such as those used in lithography for writing on objects with the aid of electron beams, the elimination of off-axis image aberrations is decisive. The aim is always to set up and adjust, in its entirety, the system comprising the imaging lens system and the corrector such that the efficiency-limiting image aberrations of the entire system are eliminated or substantially minimised, the corrector having the function of, on one hand, achieving, by negative image aberration coefficients, an elimination or at least a reduction and on the other hand causing no increase of disadvantageous image aberration coefficients.

[0003] The aim is always to set up and adjust, in its entirety, the system comprising the imaging lens system and the corrector such that the efficiency-limiting image aberrations of the entire system are eliminated or substantially minimised, the corrector having the function of, on one hand, achieving, by negative image aberration coefficients, an elimination or at least a reduction, and on the other hand not causing an increase, of disadvantageous image aberration coefficients.

[0004] The object of the present invention is to provide an electron-optical corrector, with the aid of which, in addition to the first order, first degree chromatic aberration, all third-order image aberrations, such as spherical aberrations, field curvature, off-axis astigmatism, off-axis coma, and distortion are eliminated in such a way that no additional off-axis image aberration is generated.

[0005] This object is achieved according to the invention by means of a corrector which, symmetrical about the central plane, has the following construction:

[0006] In the direction of the beam path, a hexapole S₁ of length l1 is first positioned, followed by a circular lens R₁, then a hexapole S₂ of length l₂, and then a circular lens R₂, which is followed by a third hexapole S₃,with the same strength as the hexapole S₁ and twice the length l₃=2l₁ thereof, the separation of the two circular lenses R₁ of the same strength, being 2f of the focal length, the hexapole S₂ being positioned in the principal focus between the two circular lenses R₁, R₂, and the distance of the circular lens from the first hexapole S₁ being chosen such that the inner plane of the S₁ comes to lie in the front principal focus of the circular lens that is positioned downstream, and the centre of the hexapoles S₂ and S₃ is positioned in the focal plane, and this is finally followed by further elements of the corrector, which are set up symmetrically to the central plane Zm of the hexapole S₃.

[0007] To produce the required telescopic Gaussian ray path, the distance between adjacent circular lenses is in each case 2f, f representing the focal length of the circular lens. The hexapoles S₂, S₄ and S₃ are to be positioned with their centres, in each case, being positioned in the principal focus of the circular lenses. The hexapoles S₁ and S₅ are assigned such that their respective inner plane comes to lie in the focal point of the circular lens. To prevent the occurrence of second order aberrations, the strength of the hexapole lenses S₁, S₃ and S₅ is in each case of the same magnitude. In addition, S₂=S₄ is adjusted, however their strength can be chosen completely independently of S₁, S₃ and S₅. However, a symmetry with respect to the central plane Z_(m) in the set up and adjustment must always be ensured.

[0008] To eliminate spherical aberrations, field curvature and off-axis astigmatism (all third-order aberrations), the intensity of the hexapoles S₁ and S₂ are available and parameters that can be chosen freely and independently of one another. By corresponding setting, two of the aforementioned image errors in the entire system can be eliminated. For purposes of lithography, it is most important td eliminate off-axis astigmatism as well as the field curvature. Although the third-order spherical aberration of the corrector is negative and leads to a reduction of the overall aberration, however, in the most general cases it does not become zero. By means of an appropriate spatial positioning in the overall system and corresponding choice of distance from the adjacent lenses, it can be achieved for a particular enlargement that the third-order spherical aberration of the entire system can be compensated.

[0009] In addition to the elimination of the aforementioned image aberrations, the corrector itself is also free of third-order distortion and coma and of first-order, first degree off-axis chromatic aberrations, so that when the corrector is installed in a correspondingly aberration-free electron-optical system the entire system also remains free of these aberrations. For a magnification unequal to 1, theoretical considerations require that a system free of off-axis coma and distortion must consist of at least four lenses.

[0010] The decisive advantages of the proposed corrector consist principally in, with appropriately setting and spatial positioning of the spherical aberrations, adjusting the field curvature and the off-axis astigmatism (all third order) such that the entire system becomes aberration free, but also that further image aberrations, namely third order distortion, off-axis coma and first order, first degree/chromatic, aberration, of the (entire system are not increased by virtue of the corrector, since it is itself aberration-free.

[0011] In the most general case, the hexapoles are always aligned relative to one another in the same section. In cases where magnetic lenses are used, the magnetic field leads to an anisotrope (azimuthal component); which leads to image rotation. For aberration elimination, a rotation of the sections of the hexapoles S₂ (and S₄ which lies in the same section) occurs relative to the hexapoles S₁, S₃ and S₅, which are also aligned in a common section. The angle of rotation is determined by the magnitude of the anisotropic component determined by the magnetic field.

[0012] The rotation of the hexapoles can then take place with maintenance of the mechanical orientation by electrostatic means, if, to generate the hexapole field, a twelve-pole is used, which permits a rotation of the sections about any desired angle by corresponding repoling of the electrodes. It correspondingly becomes possible, after installation of the twelve poles, to rotate the hexapole field by the desired angle by corresponding change of the poling.

[0013] The corrector proposed above eliminates image aberrations up to third order. In a particularly preferred embodiment, two of these correctives are positioned one behind the others in series and optically connected to one another by means of a circular lens doublet. The distance of the two circular lenses 2f and the distance of the last hexapole of the first corrector to the first circular lens, and the distance between the second circular lens and the adjacent hexapole is also equal to the focal length f. By means of such a linking, an antisymmmetric beam path is obtained in both correctors, which leads to mutual compensation of the fourth-order aberrations of the two individual correctives. Since the remaining circular lenses are free of fourth order aberrations, only fifth and higher order aberrations remain. A corrector with this optical property is designated as a double astigmat.

[0014] The corrector proposed according to the invention is preferably used in a size-reducing electron-optical system, as is used in lithography, and which is used to reduce in size particular structures defined by a mask by means of the electron-optical system and impress and write them on a crystal (wafer) located in the image plane by means of the incident electrodes [sic]. The essential factor is the creation of electronic devices and integrated circuits with the smallest possible dimensions, the electron-optical systems, in comparison to light-optical images, having the advantage of being able to reproduce much finer structures because of their much smaller wavelength. Here, the corrector is brought into the beam path downstream of the projection lens located behind the object plane and, at the output side—if appropriate with the interposition of a transfer lens—the operating part (wafer) is written on via an objective in the image plane. The term objective lens can is to be interpreted broadly, and may consist of a system composed of a plurality of lenses. The proposed corrector has the particular advantage of be able to eliminate image aberrations limiting the effectiveness of electron-optical size-reducing imaging systems. A considerable improvement of the imaging quality is consequently to be expected.

[0015] Further details, features and advantages of the invention can be taken from the following descriptive part, in which a corrector according to the invention is described with reference to the drawing, wherein

[0016]FIG. 1 shows a section along the optical axis and reproduction of the Gaussian fundamental paths.

[0017] The diagrammatic illustration is kept in a schematic view. The hexapoles are here represented by S₁, their lengths by l₁ and the circular lenses by R₁. The corrector has the following construction:

[0018] At the inlet side, there is located the first hexapole S₁ of length L₁. This is followed by the circular lens doublet consisting of the circular lenses R₁ and R₂, the focal length of the two circular lenses R₁ and R₂ being identical, and their distance being chosen as equal to 2f, f being the focal length, such that the circular lens doublet in its totality generates a telescopic beam path. Between the two circular lenses R₁, R₂, the further hexapole S₂, of length l₂, is located in the focal plane. The first hexapole S₁ is located at a distance such that its inner plane comes to lie in the focal point of the downstream circular lens. The subsequent hexapole S₃ in the direction of the beam path is chosen such that in its intensity it is equal to the hexapole S₁, but of double its length l₃=2l₁. The spatial assignment takes place such that the centre plane of the hexapole S₃ comes to lie in the focal plane Z_(m) of the circular lens.

[0019] The further construction of the corrector in the direction of the beam path is symmetrical to this central plane Z_(m), so that both as regards the spatial positioning as well as the chosen pole strength, reference can be made to the previous models in order to avoid repetitions.

[0020] The above-described corrector has two freely selectable parameters, namely the strength of the hexapole S₁ (and thereby also that of the hexapoles S₃ and S₅) as well as the strength of the hexapole S₂ (and thereby also that of the hexapole S₄). These two parameters permit two of the following three image aberrations to be freely set, namely third-order field curvature, off-axis astigmatism and spherical aberrations, in such a manner that compensation of two of these image aberrations occurs. As already described, the third aberration (described by means of the third order spherical aberration) can be influenced in the desired way by appropriate choice of the geometrical parameters, and in particular the distance, so that extensive compensation of the third image aberration is possible. 

1. Electron-optical corrector for eliminating third-order aberrations, such as spherical aberrations, field curvature and off-axis astigmatism; said corrector being devoid of third-order off-axis coma, third-order distortion and first-order chromatic aberrations of the first degree, the corrector having a construction which is symmetrical about the central plane in the direction, of the linear optical axis, characterised in that a hexapole S₁ of length l₁ is first positioned in the direction of the beam path, followed by a circular lens R₁, a hexapole S₂ of length l₂, followed by a circular lens R₂, which is followed by a third hexapole S₃ with the same strength as the hexapole S₁ and twice the length l₃=2l1 thereof, the separation of the two circular leases R₁ of the same strength being 2f of the focal length, the hexapole S₂ being positioned in the principal focus between the two circular lenses R₁, R₂, and the distance of the circular lens from the first hexapole S₁ being chosen such that the inner plane of the S₁ comes to lie in the front principal focus of the circular lens that is positioned downstream, and the centre of the hexapoles S₂ and S₃ is positioned in the focal plane and this is finally followed by further elements of the corrector, which are set up symmetrically to the centre plane z_(m) of the hexapole S₃.
 2. Corrector according to claim 1, characterised in that the hexapole strengths S₁, S₂ are in each case chosen so as to eliminate the off-axis astigmatism, as well as the field curvature of the entire system, and the distance from the adjacent lenses of the entire system is chosen so that the third-order spherical aberration can be influenced to the extent of compensation.
 3. Corrector according to claim 1 or 2, characterised in that the section of the hexapoles S₁[sic], S₄ is azimuthally rotated about the optical axis with respect to the section formed by the other hexapoles S₁, S₃, S₅.
 4. Corrector according to claim 3, characterised in that the hexapoles are generated in a twelve-pole element comprising 12 electrodes or pole shoes.
 5. Electron-optical corrective according to one of the preceding claims, characterised in that two of the aforementioned correctors are positioned in series by means of a circular lens doublet, the spacing of two circular lenses corresponding and the spacing of the last hexapole of the first corrector from the first circular lens, and the spacing of the second circular lens of the circular lens doublet from the first hexapole in each case corresponding to the focal length of the circular lens.
 6. Use of an electron-optical corrector according to one of the preceding claims in a size-reducing electron-optical system, in particular lithography, comprising a projection lens located in the object plane formed in the direction of the beam path downstream of the mask that is usually to be imaged, and an objective lens connected upstream of the image plane, usually defined by a wafer, characterised in that the corrector is introduced into the beam path downstream of the projection lens in the direction of the beam path.
 7. Use of an electron-optical corrector according to one of the preceding claims in a size-reducing electron-optical system, in particular lithography, comprising a projection lens located in the object plane formed in the direction of the beam path downstream of the mask that is in general to be imaged, and an objective lens connected upstream of the image plane, usually defined by a wafer, characterised by a transfer lens between the corrector and objective lens. 